Examples of conventional video image-based object detection systems are disclosed in Patent Documents 1-7.
The conventional video image-based object detection systems operate as follows:
Specifically, a scheme disclosed in Patent Document 1 enables detection of an object having any shape and estimation of its attitude by sophisticating object detection without attaching a predefined marker elaborately designed beforehand for facilitating or speeding up detection processing. The object detection scheme such as that in Patent Document 1 employing no marker eliminates the need of a marker to be attached, whereas it poses a problem that it is relatively unreliable, has a low detection speed or is unstable. A scheme disclosed in Patent Document 2 solves such a problem by attaching a marker having a distinctive graphic pattern that is relatively visually conspicuous to an object desired to be detected. This document argues that accuracy in detection of any predefined graphic pattern can be improved. However, it poses a problem that once a graphic pattern similar to that of the predefined marker appears in a background by chance, these patterns are confused with each other. Patent Documents 3 and 4 avoid this problem by empirically designing beforehand a marker of a unique shape never anticipated to appear in a background, such as a light-emitting marker and a distinctively colored marker, respectively. A scheme disclosed in Patent Document 5 employs graphic markers of a special shape such as concentric arcs. It also employs means of constructing a graphic marker using an infrared reflector, for example, in combination. These schemes, however, increase cost of the marker, nevertheless still potentially pose a problem similar to that in the method disclosed in Patent Document 2 in a case that a pattern similar to the form of the marker appears in a background by chance or the marker is placed in circumstances, such as outdoors, containing disturbance that may impair uniqueness of the form of the marker. Moreover, the marker must be empirically designed by a method requiring skill to have a form probably unaffected by a background or turbulence, or designed by a trial-and-error process in an actual operating environment. Furthermore, since the marker may be observed with geometric distortion due to the degree of freedom in positioning a camera in imaging, it is necessary to prepare a detection scheme taking account of such distortion. In addition to an increase of computational cost for detection taking account of geometrical distortion, it is likely to increase a possibility that the distorted marker becomes similar to a background pattern by chance. The schemes disclosed in Patent Documents 6 and 7 involve a highly complicated marker of a graphic pattern that never becomes similar to the background pattern by chance. They may embed a redundant code in a marker itself for verifying identification of the marker. Whereas the schemes can significantly reduce the possibility of over-detection of a marker from a background, subtle graphical information on the marker must be visualized on a video image, which may cause misdetection of the marker, so that imaging coverage must be reduced or resolution of an imaging device must be increased, resulting in a presumable increase of cost for implementing a decoder and reduction of the detection speed.
Patent Document 1: JP-P2000-207568A
Patent Document 2: JP-P2003-223639A
Patent Document 3: JP-P2003-256783A
Patent Document 4: JP-P2005-293141A
Patent Document 5: JP-P2005-293579A
Patent Document 6: JP-P2006-190110A
Patent Document 7: JP-P1995-254037A